The present invention relates to a bandstop filter for very high frequency transmission lines having distributed constants and implemented in planar structure, more particularly in accordance with the microstrip technique, as well as to a biassing circuit for a very high frequency transistor comprising this filter.
A conventional technique for ensuring the reception of a very high frequency signal is to include in the receiver a mixer which receives on the one hand this useful very high frequency signal of the frequency f.sub.S and on the other hand a signal of the frequency f.sub.OL produced by a local oscillator and which produces a signal having an intermediate frequency f.sub.FI which is equal to the difference between the frequencies f.sub.S and f.sub.OL. This mixer must be followed by a filter which prevents the transmission of the frequencies f.sub.S and f.sub.OL and promotes the transmission of the weakest frequency f.sub.FI, that is to say a low-pass filter or at least a band-stop filter.
Such filters are included in the amplifier stage described in "Proceedings of the 4.sup.th European Microwave Conference", Montreux, September 1974, pages 97 to 100, (see FIG. 2), or in the oscillator stage described in "Proceedings of the 5.sup.th European Microwave Conference", Hamburg, September 1975, pages 296 etc (see FIG. 4). Nevertheless, when this filter is used alone, as is the case in the second paper, a very narrow band of this filter is cutoff and it is not suitable for the above-intended use. When, on the contrary, several filters are used in conjunction, it is possible that a wider band is cutoff but this is offset by the fact that other disadvantages are created, notably the fact that the overall filter thus realized becomes bulky and no accurate positioning of the filter with respect to the mixer is possible.
It is an object of the invention to provide a bandstop filter for very high frequencies which does not require such compromises which are inevitably unsatisfactory, and which is extremely compact while having a cutoff band which is sufficiently large and offering a properly defined plane of short-circuit with respect to the mixer to optimize the efficiency of this mixer.
To this end the invention relates to a bandstop filter which is characterized in that it comprises:
(a) a first filtering element intended to prevent the transmission of a first frequency band and comprised of a quarter-wave filter having electrical length equal to one quarter of the wavelength .lambda..sub.1 associated with the centre frequency of this first band, arranged at the input point of the bandstop filter and defining there a plane of short circuit, PA0 (b) a second filtering element intended to prevent the transmission of a second frequency band which is adjacent to the first band and which is comprised of an assembly of two parallel transmission paths which separate at the input point of the bandstop filter and come together again at the output point of this bandstop filter such, that the electric length of each of these two paths is equal to an odd number of times one quarter of the wavelength .lambda..sub.2, which is associated with the center frequency of the second frequency band and that the difference between these lengths is equal to an odd number of times half this wavelength. Preferably, this second filtering element is a closed loop having an electric length equal to .lambda..sub.2 and transversely connected to the very high frequency transmission line in the input and output points of the bandstop filter which are spaced on this loop at a distance equal to .lambda..sub.2 /4, and that the first filtering element is positioned within this loop.
The bandstop filter thus realized occupies in a very efficient manner very little space, since the combination of these two filtering elements and a proper choice of their dimensions render it possible to obtain the desired width of the cutoff band at one's option. In addition, connecting these two filtering elements to the same point, at the input point of the filter, defines in a unique and accurate manner the plane of short circuit of the signals whose transmission one wants to prevent, which renders the action of the filter in practice independent of the frequency in the cutoff band (20 to 30%). In order to render the band which was cutoff by this filter still wider and to eliminate the second harmonic of the center frequency of either the one or the other of the two frequency bands which were already eliminated by the first and the second filtering elements, this filter may comprise at least a third filtering element intended to prevent the transmission of a third frequency band centered around the frequency of double the center frequency of the first frequency band and comprised of a quarter-wave filter of the electrical length .lambda..sub.1 /8, which is arranged in parallel with one of the two transmission paths at a distance from the input point of the bandstop filter equal to (n-1) .lambda..sub.2 /2, and is a positive integer which is compatible with the length of the transmission path, or, alternatively, intended to prevent the transmission of a third frequency band centered around the frequency which is double the center frequency of the second frequency band and being comprised of a quarter-wave filter of an electrical length .lambda. .sub.2 /8 arranged in parallel with one of the two transmission paths at a distance from the input point of the bandstop filter equal to (n-1) .lambda..sub.2 /2, and being a positive integer which is compatible with the length of the transmission path.